1. Field of the Invention
The present invention relates generally to a coupling device which protects against tension overloads when used to connect two elements which are placed under tension, in particular those coupling devices which are used in a wellbore with downhole tools.
2. Discussion of the Related Art
In general, it has been found useful in an oil well or wellbore to use couplers placed between an electrical signal conducting cable and downhole tools to protect against tension overloads on the cable. In operations for producing hydrocarbon fluids difficulties arise when the downhole tools become stuck in the borehole which can cause a tension overload when the cable is used to remove the tool from the wellbore. Coupling devices or connectors which break the connection between the cable and downhole tools prevent damage to the cable and the tool due to the tension overloads. Various coupling devices are known in the prior art for this purpose. However, as is well known in the art, such couplers have had many disadvantages and many problems associated therewith and which are described hereinafter.
A coupler or connection between a cable or conduit and a downhole tool serves three principle functions: first to securely fasten the cable or conduit to the downhole tool; second, to provide a means for preferential breaking of the connection between the cable and the tool to avoid damage to the cable and tool and protect against tension overloads; third, to allow electric wires, fluids, gases or mechanical linkages to pass through the coupler without contamination by external materials. The function of providing a means of preferential breaking of the connection at a predetermined tensile load level is important because it is fairly common for cable-supported tools to become stuck in boreholes. This function permits the operator to preserve the majority of the cable in the event the tool string becomes hopelessly stuck in the borehole. It should be noted however that the coupling or connection must be strong enough to support the static weight of the tool string suspended from the cable. The coupling or connection must also be strong enough to support the weight of any additional loads imparted to the tool during normal operation or in reasonable efforts to free the tool string should it become stuck.
Known coupling devices or connectors employing breakaway elements suffer numerous disadvantages. For example, prior art devices are unreliable in their failure points. Many prior art devices are difficult to inspect to insure that the weak point element has not been partially stressed even though it remains unbroken and will thus have an unpredictable failure point. Many prior art devices are also complex in their structure and do not have the advantage of being easily adjustable or reusable after breakage.
Some prior art devices have made use of a shear pin in a position perpendicular to the longitudinal axis of the coupler as the weak point element of the coupler or connection. However, there are numerous problems associated with the use of a shear pin in a breakaway coupling device. Shear pins do not break clean under stress and therefore have jagged edges which tear up the coupling device after breakage. When a shear pin breaks jagged and tears up the coupler or connector, the coupler or connector cannot be used again. Also, the repeated use of a device using shear pins may cause the device to become unreliable in its failure point due to changes caused in the device by the shear pins themselves. Another problem occurs when the shear pin only partially shears and the coupler or connector cannot then be pulled apart. Therefore, prior art devices using a shear pin are not easily reusable.
In addition to the drawbacks previously noted, prior art coupling devices have also had numerous problems with contamination of the internal passages by external materials such as fluid and solids from the borehole. Since such things as electrical wires and cables must run through the coupler, it is essential that no moisture or contamination leaks into the interior of the coupler. Many prior art devices use a face seal which may leak when subjected to large loads which stretch the coupler but do not break the connection.
Accordingly, prior to the development of the present invention, there has been no downhole breakaway coupling device adapted for use in a wellbore which: is noncomplex in structure and is simple and economical to use; provides a high degree of predictability of tensile and load strength and breakpoint; provides for easy adjustability of the breakpoint; provides a coupler which is easily reusable after the failure point of the coupler is reached; allows electric wires, fluids, gases or mechanical linkages to pass through the coupler without contamination by external materials; and provides a retrieval notch whose function is to aid in retrieving downhole tools stuck in a borehole after breakage of the coupler.